How can Obamacare be so terrible it's worth destroying the US economy to stop it?

The Affordable Care Act is a large and complex piece of legislation. It’s certainly got its strengths and weaknesses. If it’s really terrible, then soon after it takes effect, that will become obvious and we can either fix what is wrong with it or hit the reset button, repeal it entirely, and just go back to what we had before it was passed. But how on Earth is one law, flawed or not, worth shutting down the US government for 3 weeks now and, if the Tea partiers in the house have their way, deliberately defaulting on the US debt for the first time in history.

Here’s what a deliberate default will likely do. How can we take any lawmaker seriously when they WANT to do this?